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Activity: Modeling Evolution

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Bacteria are single-celled, small, simple organisms. They do not have specialized compartments (organelles) inside their cells. However, bacteria do perform essential roles in the environment, from decomposition in the soil to digestion in the human gut (Fig. 1.2 D; Fig. 1.5). Bacteria are often used to model life cycles and evolution because they go through many generations in a relatively short time. In this activity, you will model how a bacteria population is affected by exposure to an antibiotic.

<p><strong>Fig. 1.5.</strong> (<strong>A</strong>) Scanning electron microscope image of a disease-causing bacteria, <em>Escherichia coli</em></p><br />
<p><strong>Fig. 1.5.</strong>&nbsp;(<strong>B</strong>) Compound microscope image of blue-green bacteria or cyanobacteria</p><br />


Bacteria normally die when exposed to an antibiotic, such as penicillin. However, some bacteria have developed heritable traits that make them resistant to antibiotics. The primary cause of antibiotic resistance in bacteria is genetic mutation. When a mutation allows a bacterium to survive in the presence of an antibiotic, the surviving bacteria pass on their antibiotic resistance mutation to their offspring when they reproduce.

 

In this activity, the starting bacteria population includes (1) typical bacteria that die when exposed to antibiotics and (2) mutated bacteria that are antibiotic resistant. The mutated bacteria have a higher chance of survival when exposed to an antibiotic. The paper clips in this activity represent bacteria. The term phenotype is used to describe the physical traits displayed by an organism. All phenotypes are the expression of genetic information in an individual’s DNA molecules. The plastic-coated paper clips represent the typical bacteria phenotype, and the silver paper clips represent bacteria that have undergone a mutation that gives them antibiotic resistance.

 

Model Assumptions

  • Typical bacteria have a 1-in-6 chance of surviving exposure to an antibiotic.
  • Mutated bacteria have a 5-in-6 chance of surviving exposure to an antibiotic.
  • Both typical and mutated bacteria produce offspring of the same type. This means that typical bacteria will produce typical bacteria and mutated bacteria will produce mutated bacteria.

 

Paid
Video

Workshop: Modeling Evolution

Paid
Teacher Guide

Teacher Guide: Modeling Evolution

 

Materials

  • Table 1.4
  • Fig. 1.6
  • Paper clips
    • 50 plastic-coated (typical bacteria)
    • 50 regular silver (mutated bacteria)
  • Six-sided die
  • Pens or pencils (two colors)

<p><strong>Fig. 1.6.</strong> Number of typical and mutated bacteria over generations</p><br />

Procedure

  1. Start with a population of 20 bacteria, 18 typical and two mutated. Record the starting bacteria population for both typical and mutated bacteria in Table 1.4 in the column labeled “At start of generation.”
     
  2. The entire population of bacteria will be exposed to an antibiotic. You will simulate this event by rolling the die for each individual bacterium (paper clip) to see if the bacterium survives antibiotic treatment.
    1. For typical bacteria, which have a 1-in-6 chance of surviving exposure to an antibiotic, survival and reproduction happen only when a 1 is rolled. Any other roll will lead to death (see Table 1.3).
    2. For mutated bacteria, which have a 5-in-6 chance of surviving exposure to an antibiotic, survival and reproduction occurs in rolls of 1–5. Death only occurs when a 6 is rolled (see Table 1.3).
Table 1.3. Dice roll determining bacteria survival
Bacteria Dice Roll
1 2 3 4 5 6
Typical
(coated paper clip)
Survives Dies Dies Dies Dies Dies
Mutated
(silver paper clip)
Survives Survives Survives Survives Survives Dies

 

  1. Predict the number of typical and mutated bacteria that will constitute your population of bacteria at the end of five generations.
     
  2. For each individual bacterium, roll the die.
    1. Determine if the bacterium survives by consulting Table 1.3.
    2. When a bacterium dies, remove it from the population by setting it aside.
    3. Record the number of bacteria that died after antibiotic treatment in the “Dead” column in Table 1.4.
    4. Record the number of bacteria that survived after antibiotic treatment in the “Survivors” column in Table 1.4.
       
  3. The surviving bacteria reproduce. Bacteria divide in half when they reproduce, in a process called binary fission. Each surviving bacteria becomes two bacteria. In Table 1.4, use the number of survivors from generation 1 to calculate and record the total number of bacteria after each surviving bacteria reproduces in the “Reproduction” column in Table 1.4. (Hint: Multiply the number of surviving bacteria by two.)
     
  4. Write the number of bacteria in your “Reproduction” column at the end of generation 1 in the column “At start of generation” for generation 2.
     
  5. Repeat steps 2–4, filling in Table 1.4 for another four generations.
     
  6. Graph your results for both typical and mutated bacteria in Fig. 1.6. Use the numbers in the “At start of generation” column. Use a different color for each type of bacteria.
     
  7. Optional Part A: If there is time, run the experiment again.
     
  8. Optional Part B: If there is time, continue to model the experiment through another five generations. Before you start, predict the number of typical and mutated bacteria that will be present at the end of ten generations.

 

Activity Questions: 
  1. Compare your final typical and mutated bacteria numbers with the class data set. How are your findings similar or different? Why?
     
  2. Did your observations match your predictions? Why or why not?
     
  3. On average, how do the proportions of typical and mutated bacteria change in the population over time?
     
  4. What would have happened if all of the mutated bacteria died during the first generation?
     
  5. What was necessary to model evolution? (Hint: Think about how the game would work if all the paper clip bacteria looked the same.)
     
  6. This activity simulated a selective process that resulted in a shift of gene variation within a population over a short time scale. How do you think a shift in the proportion of genes in a population could lead to the evolution of a new species?
     
  7. Explain why evolution happens to a whole population rather than to a single individual.
     
  8. Animals can develop illnesses from bacterial infections. Antibiotic medications are sometimes prescribed to treat these infections What do you think might happen to a population of bacteria that is exposed frequently to such antibiotics?
     
  9. Before the widespread use of antibiotics, there were only low levels of antibacterial resistance. As antibiotic use has grown, so has the number of antibiotic resistant bacteria. Why do you think this has occurred?
     
  10. During a bacterial infection, exposure to antibiotics helps kill off bacteria. However, the antibiotic must be administered for a relatively long period of time. What might happen if someone was prescribed antibiotics but did not complete their full course?
     
  11. If you did Procedure 10, optional part B (continue to model the experiment through another five generations), how did the first five generations of the model compare to the second five generations of the model?

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.